Seawater is becoming more acidic as the oceans absorb carbon dioxide from the atmosphere, where rising concentrations are the cause of global warming. Many scientists are concerned about the impact of acidification on marine life.

Williamson said he was frustrated by the ruling: “Ipso’s overall message that ocean acidification is just a matter of opinion, not hard-won, testable knowledge is pernicious, with serious policy consequences.”

Another researcher, whose study was cited by Delingpole as “heavy-fire support” for scepticism about acidification, criticised the Ipso decision.

Prof Howard Browman, at the Institute of Marine Research, Norway, said: “During these times of fake news in a post-fact world, it is essential that organisations such as Ipso are able to differentiate between a scholarly exposition that is based upon the best available information [Williamson’s complaint to Ipso] and an opinion piece.”

Carol Turley, a marine expert at the Plymouth marine laboratory in the UK was also critical, calling Ipso’s decision “deplorable”.

Fraser Nelson, editor of the Spectator, said: “It’s odd that, in a nation which cherishes free speech, so many of those who disagree with articles feel the need to report an author to a regulator rather than write in and argue their own case. The Spectator is proud to abide by [Ipso’s] editors’ code: every fact has to be correct. But there is no such thing as a ‘correct’ opinion. Proper science invites challenge and argument, and there have been plenty of both in the pages of The Spectator since 1828.” Williamson said he did contact the Spectator in August to propose a response but got no reply.

Delingpole, who writes for controversial rightwing news site Breitbart, was censured by the Australian Press Council in 2012 after he quoted an anonymous source who compared the windfarm industry to a paedophile ring. He has dubbed greens “eco-nazis” and in another article he ended a long list of people and groups supporting action on climate change by writing: “Truly there just aren’t enough bullets!”

Delingpole’s article, published in April 2016, said: “Ocean acidification – the evidence increasingly suggests – is a trivial, misleadingly named, and not remotely worrying phenomenon which has been hyped up beyond all measure for political, ideological and financial reasons.”

It also said: “Why, between 2009 and 2014, did Defra spend a whopping £12.5m on an ocean acidification research programme when the issue could have been resolved, for next to nothing, after a few hours’ basic research?”

Williamson was the scientific coordinator of that research programme and told Ipso that the idea that “marine life has nothing whatsoever to fear from ocean acidification”, as Delingpole claimed, was “contrary to the totality of the peer-reviewed scientific literature on the topic”.

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But in its ruling, Ipso said the article “made clear that many were concerned by the possible consequences of ocean acidification, and it was not misleading for it to describe the alternative point of view”. Ipso did find the article contained some inaccuracies, such as the details of a chart it referred to, but that these were not significant.

“As an individual, Delingpole is entitled to hold such views, even if incorrect,” said Williamson. “But it is irresponsible journalism for such claims to be published without credible supporting evidence.”

Prof Paul Pearson, at Cardiff University in the UK, said: “The reality of ocean acidification and its likely long-term effects have been thoroughly documented and extensively published in the world’s most respected scientific journals for many years.”

Pearson added: “That is not to say that the body of research is either fully complete or above all criticism – no science is – but the public deserves to know that the work they have funded has by now firmly established that the problem of ocean acidification is undoubtedly real and the consequences for marine life are likely to be very serious unless action is taken fast to cut emissions.”

Attacking climate change scepticism, Prof Sam Dupont, at the University of Gothenburg, Sweden, said: “I am a sucker for conspiracy theories but you have to ask yourself what is the most plausible: hundreds of scientists from more than 50 countries working secretly together to promote a false idea, or merchants of doubt with financial and political interests at stake working very hard to undermine the scientific evidence.”